The present invention relates generally to thin film deposition technology, and more particularly to thin film deposition using a shadow mask.
Thin films are used in a variety of applications. For example, thin films are often used in the production of Micro-Electromechanical Systems (MEMS) and Micro-Opto-Electromechanical Systems (MOEMS). In such applications, one or more thin film layers are typically applied to a substrate, such as a silicon or polysilicon wafer. The thin film layers can be used to form various electrical, mechanical, and optical features on the substrate.
One technique for depositing thin films uses a shadow mask to control the deposition of a thin film material onto the substrate. Specifically, various openings are formed in the shadow mask corresponding to various features to be applied to the substrate. The shadow mask is typically made from stainless steel, nickel, or copper, and may include multiple material layers (e.g., a thick material layer for defining gross features and a thinner material layer for defining fine features). The various openings may be formed in the shadow mask, for example, using a photochemical etching process. The shadow mask is placed in front of the substrate, typically within a vacuum chamber. The shadow mask and substrate are then exposed to vaporized thin film material, which is at extremely high temperature. The vaporized thin film material is deposited onto the substrate only where the substrate is exposed through the openings in the shadow mask, essentially in a xe2x80x9cline of sightxe2x80x9d from the source of the vaporized thin film material.
When exposed to the high-temperature vaporized thin film material, both the shadow mask and the substrate tend to expand due to thermal expansion. The shadow mask tends to expand faster than the substrate. One reason for this is that the shadow mask material typically has a higher coefficient of thermal expansion than the substrate material. Another reason for this is that the shadow mask typically heats up faster than the substrate, both from the heat of the vaporized material and from radiant heat, due to its placement in front of the substrate.
As the shadow mask expands, the shadow mask openings move relative to the substrate. The amount of movement of a particular opening depends upon its distance from the center of the shadow mask, with the amount of movement tending to increase as the distance from the center increases. FIG. 1 demonstrates this position-dependent movement of the openings in an exemplary shadow mask 100, where the center opening is substantially unaffected but the four openings at the periphery of the shadow mask move outward from the center. This expansion of the shadow mask relative to the substrate tends to cause xe2x80x9cdriftingxe2x80x9d or xe2x80x9crun outxe2x80x9d of the various features deposited on the substrate, which can cause, among other things, uneven deposition of the thin film material and misshapen features, and can limit the size and proximity of the various features.
In accordance with one aspect of the invention, the shadow mask material is selected so that the expansion characteristics of the shadow mask during thin film deposition closely match the expansion characteristics of the substrate. The shadow mask material is typically one with a low coefficient of thermal expansion (CTE), in part because the substrate typically does not expand greatly during the thin film deposition. The shadow mask material must typically meet additional criteria, such as mechanical strength, feature quality, and dimensional accuracy criteria.
In accordance with another aspect of the invention, a method for thin film deposition involves providing a shadow mask material having expansion characteristics that closely match expansion characteristics of a substrate during a thin film deposition process, forming a shadow mask from the shadow mask material, and using the shadow mask to deposit a thin film material on the substrate using the thin film deposition process. The shadow mask material typically has a low coefficient of thermal expansion, and typically also has sufficient mechanical strength, feature quality, and dimensional accuracy characteristics. The shadow mask material may be a nickel alloy material such as INVAR(TM) or KOVAR(TM), silicon, or any of a variety of metals such as tungsten, molybdenum, zirconium, hafnium, rhenium, tantalum, iridium, and ruthenium. Forming the shadow mask typically involves forming openings in the shadow mask corresponding to various features to be applied to the substrate through a process of thin film deposition. Using the shadow mask to deposit a thin film material on the substrate using the thin film deposition process typically involves placing the shadow mask in front of the substrate and exposing the shadow mask and substrate to a vaporized thin film material.
In accordance with another aspect of the invention, a shadow mask for thin film deposition includes a material having expansion characteristics that closely match expansion characteristics of a substrate during a thin film deposition process and at least one opening formed in the material corresponding to at least one feature to be applied to the substrate during the thin film deposition process. The material typically has a low coefficient of thermal expansion, and typically also has sufficient mechanical strength, feature quality, and dimensional accuracy characteristics. The material may be a nickel alloy material such as INVAR(TM) or KOVAR(TM), silicon, or any of a variety of metals such as tungsten, molybdenum, zirconium, hafnium, rhenium, tantalum, iridium, and ruthenium.
In accordance with another aspect of the invention, a product is formed by the process of forming a shadow mask from a shadow mask material having expansion characteristics that closely match expansion characteristics of a substrate during a thin film deposition process, placing the shadow mask in front of the substrate, and exposing the shadow mask and substrate to a vaporized thin film material as part of the thin film deposition process. The product may be a MEMS product or a MOEMS product.